A voltage controlled oscillator is commonly constructed using a cross-coupled negative resistance cell 10 and a resonant tank circuit 11 as shown in FIG. 1. The output conductance of the cross-coupled cell alone may be expressed approximately by the following equation which is valid over a very broad frequency range: EQU G.sub.O =-gm/2 (1)
where gm is the transconductance of either transistor in the cross-coupled cell.
The fact that the output conductance is negative will, in general, guarantee oscillations at any frequency for which the resonant tank presents a conductance with a smaller magnitude (thus resulting in a net negative conductance). The amplitude of the oscillations, at a given frequency, will be proportional to the magnitude of the resonator complex impedance at that frequency. The purpose of the resonant tank then is to present the negative resistance cell with both a small conductance (usually over a very broad frequency range), and a large complex impedance (usually over a narrow band of frequencies, the centre of which is controlled by varying the value of the resonator capacitance through an external control voltage).
The primary performance criterion for a VCO is the phase noise expressed by the following equation: ##EQU1## where ELG is the excess loop gain,
Q is the resonator Q PA1 .phi. is voltage-current phase delay PA1 f.sub.0 is center frequency of oscillation PA1 .DELTA.f is frequency offset from center PA1 V.sub.n is tank-referred noise voltage PA1 V.sub.s is tank-referred signal
From Equation (2) it is apparent that to minimize the phase noise it is desirable to obtain a high-Q resonant tank circuit. (The Q of the tank is defined as the ratio of the susceptance of either reactive component at resonance to the net conductance of the tank). Typical integrated circuit (IC) technologies do not in general allow the realization of high-Q components on the chip, and so it is common to make use of external, high-Q elements such as inductors and voltage-variable capacitors (varactors) which are connected to the negative resistance cell through the IC bond pads, bond wires, package leads and board traces, as shown in FIG. 2. These so-called board and package parasitics 12 (hereinafter referred to simply as package parasitics) result in spurious resonances (frequencies at which the tank impedance is high) which can cause the VCO to oscillate at frequencies other than the intended frequency.
Typically, elaborate and costly packaging and/or IC processing techniques, or the use of lower-Q (and hence noisier) external or internal tank circuits are required to overcome the problem of spurious oscillations in the design of such IC VCOs.